Linear low density polyethylene (LLDPE) is an ethylene polymer prepared using a coordination catalyst in the same manner used in preparing linear high density polyethylene (HDPE), and is actually a copolymer of ethylene and at least one higher alpha-olefin. The expression "linear polyethylene" thus includes those which have from 0% to about 30% of at least one higher alpha-olefin of 3 to 12 carbon atoms copolymerized with the ethylene, and it will be understood that the linear polyethylenes which contain a positive amount of at least one higher alpha-olefin will, because of the presence of the higher alpha-olefin in the polymer chain, have a lower density than those which contains none of the comonomer.
It is known from EPO 85 101380.5 that linear low density polyethylene (LLPDE) can be made into fibers by melt-spinning and that even very fine fibers can be produced. This EPO publication is incorporated herein by reference.
It is known from U.S. Pat. No. 4,578,414 that wettable fibers of polyethylene, including LLPDE, can be prepared from polyethylene having incorporated therein certain wetting agents.
The manufacture of LLPDE is disclosed, e.g., in U.S. Pat. No. 4,076,698, which discloses that the LLPDE can be extruded as a monofilament and then cold drawn. It is shown as being a copolymer of ethylene and at least one other higher alpha-olefin. The density of the LLPDE is dependent on the chain length and amount of the higher alpha-olefin in the copolymer.
Whereas LLPDE has been introduced in the market place as a polymer suitable for making fibers, it has been determined that not all versions and varieties of LLPDE are entirely adequate for commercial production of spun-bonded fibers and the fabric strength of spun-bonded fabrics has generally been significantly less than fabrics made from spun-bonded polypropylene fibers. It is disclosed in recent U.S. Pat. No. 4,644,045 that there is a very narrow critical range of LLPDE properties which is suitable for making spun-bonded webs; this '045 patent is incorporated herein by reference, especially in reference to its disclosure of ways of measuring various polymer properties.
It is recognized in the art of making LLPDE polymers that the density of the LLPDE is affected by the amount and kind of olefin comonomer which is copolymerized with the ethylene and, to some extent, by the process conditions and catalyst used. A given mole % of, say, propylene in the copolymer will reduce the density of the polyethylene less than the same mole % of a higher olefin comonomer. The MFR (Melt Flow Rate) is also affected to some degree by the kind and amount of olefin comonomer in the copolymer, and is also affected to some extent by the coordination catalyst used, the polymerization conditions, and/or by an telogens or chain regulators or other reactants which may be present during polymerization.
It is also recognized in the art that there are important distinctions between linear polyethylenes (which include LLDPE polymers), and branched-chain ethylene polymers, which are made using a free-radical catalyst and are generally referred to as LDPE (low density polyethylene), and were also known in the past as ICI-type polyethylene and as HPPE (high pressure polyethylene). This disclosure deals with linear polyethylenes.
We have now found that blends of linear polyethylene, especially LLDPE, having certain properties, are surprisingly well suited for making spun-bonded webs and yield products having strengths more competitive with those attainable in spun-bonded webs of polypropylene, and furthermore the blends are also particularly well suited for making other forms of fibers such as staple fibers and products made from such other fibers.